While the system of equations (REF ) and () has been presented in the context of the nonlinear optics
of nematic liquid crystals, they are more general than this. The same system of equations arises in the optics of nonlinear
thermal optical media [1]}, [2]}, for example lead glasses [3]}, [4]}, [5]}, and certain photorefractive crystals
[6]}. A similar system of equations also arises in simplified models of fluid turbulence [7]} and quantum
gravity [8]}.
